1. Field of the invention
The present invention relates to a method for operating a high pressure discharge lamp containing a rare gas, mercury, metal halide, or other filler, and relates particularly to an operating method and operating apparatus whereby a high frequency alternating current component is supplied to a discharge lamp to control arc curvature.
2. Description of the prior art
An operating method for a high pressure discharge lamp according to related technology is described, for example, in the Proceedings of the 10th Anniversary session in 1983 of Tokyo branch of Illuminating Engineering Institute of Japan. The operating method described in these Proceedings operates a lamp by supplying a low frequency (several hundred hertz), rectangular wave ac current to the lamp. A problem with this operating method is that convection causes an undesirable curvature in the discharge arc when the discharge lamp is operated in a non-upright, e.g., horizontal, position, or more specifically, when the arc gap is horizontal. This curvature of the discharge arc creates a higher heat load in the top part of the discharge space, thus deteriorating the discharge envelope and shortening the service life of the lamp.
Various operating methods intending to suppress this discharge arc curvature have been proposed. One of these methods, as disclosed in Japan Examined Patent Publication (kokoku) 2-299197 (1990-299197), proposes to select a frequency of the voltage or current supplied to the lamp as a means of exciting acoustic resonance inside the discharge lamp envelope as a means of suppressing discharge arc curvature caused by convection. This specification further describes that modulating the operating frequency is advantageous as a means of expanding the frequency range that can be used for operating a lamp with a stable arc free of curvature, and as a means of compensating for ballast tolerance and the discharge tube manufacturing tolerance.
Another specification, disclosed in Japan Examined Patent Publication (kokoku) 7-9835 (1995-9835), teaches a method for supplying to a discharge lamp a unidirectional (dc) current having a superposed high frequency ripple-type ac current component. This ripple-type ac current component causes instantaneous lamp power fluctuations, which have the effect of inducing an acoustic resonance to suppress discharge arc curvature. This specification also teaches a method of frequency modulating the high frequency ripple ac component as a means of increasing the bandwidth of frequencies that can be used to obtain a straight, stable arc.
With the method described in Japan Examined Patent Publication (kokoku) 2-299197 (1990-299197), the frequency of the supply current used to operate a discharge lamp is selected for the purpose of inducing acoustic resonance inside the discharge envelope as a means of suppressing discharge arc curvature caused by convection. While this method achieves stability in the high luminance arc center (high temperature arc area), the surrounding low luminance arc area (low temperature arc area) can be unstable. This is described in further detail below with reference to FIG. 1.
Shown in FIG. 1 are the electrodes 100 determining the arc gap, the high luminance arc center 101, and the low luminance arc periphery 102 surrounding the high luminance arc center 101. As shown in FIG. 1, the high luminance arc center 101 is straight and stable. The low luminance arc periphery 102, however, exhibits unstable behavior fluctuating both vertically and horizontally with an appearance similar to a candle wavering in the breeze. It should be noted that this instability (wavering) of the low luminance arc periphery is not suppressed using the frequency modulation technique taught by Japan Examined Patent Publication (kokoku) 2-299197 (1990-299197). Details of topics with related conventional operating methods are described next below with reference to a discharge lamp comprised as shown in FIG. 2.
Referring to FIG. 2, a transparent quartz envelope 1 is sealed at both ends by seals 6a and 6b. A metal foil conductor 3a and 3b made from molybdenum is bonded to seals 6a and 6b, respectively. An electrode 2a, 2b and an external lead 4a, 4b also made from molybdenum are electrically connected to metal foil conductor 3a and 3b, respectively.
Each electrode 2a, 2b comprises a tungsten rod 7a, 7b and a tungsten coil 8a, 8b. The coil 8a, 8b is electrically bonded by welding to the end of the corresponding tungsten rod 7a, 7b, and functions as a radiator for the electrode 2a, 2b. The electrodes 2a and 2b are disposed inside the envelope 1 so that the gap therebetween, i.e., the arc gap, is approximately 3.0 mm.
The envelope 1 is roughly spherical with an inside diameter of approximately 10.8 mm and an internal volume of approximately 0.7 cc. The envelope 1 is filled with 4 mg of an iodide of indium (indium iodide, InI) as a filler; 1 mg of holmium iodide (HoI.sub.3) as a rare earth iodide; 35 mg of mercury as a buffer gas; and 200 mbar of argon as an inert gas for starting.
Concerns relating to generating an arc with a typical sine wave ac supply are described next below.
A high pressure discharge lamp comprised as described above is typically driven by supplying a sine wave shaped ac current supply from external leads 4a, 4b, thus energizing the arc gap in a horizontal position to output 200 W. As taught in Japan Examined Patent Publication (kokoku) 2-299197 (1990-299197), the frequency f was then adjusted between 10 kHz and 20 kHz and the arc was observed to select the frequency range acoustically straightening the arc. Observations showed that the high luminance arc center was straight and stable with a currency supply between 14 kHz and 16 kHz. More specifically, acoustic resonance eliminating discharge arc curvature was confirmed to be excited with a currency supply between 14 kHz and 16 kHz. However, careful observation of the arc resulting from this supply current frequency band also showed irregularly oscillating, unstable movement in the low luminance arc periphery as described above with reference to FIG. 1.
The results of these arc observations at various supply frequencies f are shown in FIG. 4. The white areas in FIG. 4 indicate a frequency band at which arc is stable in both the arc center and arc periphery, and the arc is straight. Shaded areas indicate frequencies at which the arc center is stable and straight, but the arc periphery is unstable. It should be noted that this oscillation is extremely irregular; there are cases when oscillation continues uninterrupted, and there are also cases when oscillation occurs only a few times per hour or less.
It should be further noted that while the frequency modulation method taught by Japan Examined Patent Publication (kokoku) 2-299197 (1990-299197) is able to suppress this oscillation of the arc periphery to a certain degree, this suppression simply reduces the number of oscillations and does not completely eliminate the oscillations.
Concerns relating to exciting an arc by supplying a rectangular wave current with a superposed high frequency ripple signal to the lamp are described next below.
Referring to the teaching of Japan Examined Patent Publication (kokoku) 7-9835 (1995-9835), a current comprising a high frequency ripple signal r superposed to a 100 Hz rectangular wave current k as shown in FIG. 5 was supplied to operate a discharge lamp as shown in FIG. 2. (It should be noted that the frequency fr of the high frequency ripple signal r inducing acoustic resonance must be twice the supply current frequency when a normal sine wave ac supply is used for operating because the lamp power frequency must be the same as when the lamp is operated with a sine wave ac supply.) Using the lamp shown in FIG. 2, the arc was again observed while varying the frequency fr of the high frequency ripple signal between 28 kHz and 32 kHz, the frequency at which acoustic resonance eliminating arc curvature occurs. Based on the teaching of Japan Examined Patent Publication (kokoku) 7-9835 (1995-9835) that the arc stabilization frequency band increases as the ripple becomes stronger, tests were conducted with the amplitude Ir of the high frequency ripple signal r set so that the ripple level, i.e., modulation depth (defined here as the amplitude Ir of high frequency ripple signal r divided by twice the effective lamp current) was substantially constant at 0.82. Observations showed that while the arc center was straight and stable throughout the 28 kHz to 32 kHz frequency band, irregular oscillation was present in the arc periphery.
The inventors of the present invention then measured the ripple level at which the arc periphery begins to stabilize at a particular frequency fr of a high frequency ripple signal r when the ripple level is varied by gradually varying the amplitude Ir of high frequency ripple signal r. The result is shown in FIG. 6. Operating points within the shaded area above line 6A in FIG. 6 are where the arc periphery is unstable (irregular oscillation); during operation under the curve, the arc periphery is stable (no oscillation).
As shown by these results, the frequency band at which a completely stable arc is achieved in both the arc center and the arc periphery narrows as the ripple level increases, i.e., as the amplitude Ir of the high frequency ripple signal r increases. As shown in FIG. 7, for example, a stable arc is obtained throughout the full frequency band 7A from 28 kHz to 32 kHz at a steady ripple level of 0.4. At a steady ripple level of 0.7, however, a stable arc is achieved only in frequency bands 7B and 7C, covering approximately 50% of the full band. When the ripple level is approximately 0.8 or above, the arc oscillates across the full frequency band. This result, it should be noted, is different from the teaching of Japan Examined Patent Publication (kokoku) 7-9835 (1995-9835) that the stable arc frequency band increases as the ripple level increases.
The result shown in FIG. 6 also means that as the ripple level increases in a high frequency ripple signal r of a constant frequency fr, i.e., as the amplitude Ir of the high frequency ripple signal r increases, the tolerance range to the ripple level at which oscillation starts in the arc periphery decreases, and arc instability tends to increase. This is described with reference to FIG. 8.
When the frequency fr of the high frequency ripple signal r is a constant 30.2 kHz as shown in FIG. 8, for example, the tolerance range to the start of arc periphery oscillation at a ripple level of 0.4 has a width equivalent to approximately 0.35 ripple level as shown by 8A in FIG. 8. The tolerance range at a ripple level of 0.7, however, narrows to approximately 0.05 ripple level as shown by 8B. This tendency applies to all frequencies fr.
The ripple level at which oscillation of the arc periphery begins (curve 6A in FIG. 8) may drop in a manner narrowing the stability range of the arc periphery (curve 6B, FIG. 8) as a result of manufacturing variations in the lamp and aging. To avoid such oscillation of the arc periphery, the amplitude Ir of high frequency ripple signal r must be set to a level lower than the ripple level at which arc periphery oscillation begins.
A ripple level between 0.5 to 0.6 is considered desirable because the frequency band through which a stable arc can be achieved is relatively wide, and the tolerance to a ripple level at which arc periphery oscillation begins is also relatively great.
The experimental results shown in FIG. 9, however, indicate a separate problem. The graph in FIG. 9 shows a relationship between ripple level and the amount of arc curvature when the frequency fr of the high frequency ripple signal r is a constant 30.2 kHz as above. This graph shows the ripple level on the horizontal axis, and the amount of arc curvature (distance from a center line joining the electrodes to the highest luminance point of the arc). As the value on the vertical axis rises, arc curvature increases (the arc rises to a greater height). FIG. 9 thus shows that arc curvature decreases as the ripple level increases, and that to achieve the smallest arc curvature, the ripple level should be 0.65, or preferably 0.7, or greater. To obtain a straight arc, the ripple level should be 0.5 or greater, and even more preferably should be 0.7 or greater.